Rotational Resonance Tickling:  Accurate Internuclear Distance Measurement in Solids

Accurate distance measurements between pairs of nuclear spins can provide detailed information about molecular structure in the solid state. The rotational resonance (R2) technique, in particular, has been used to measure internuclear distances in a variety of compounds, often between pairs of 13C nuclei where the outer range is 5−6 Å. Recent studies have revealed that the spin dynamics at rotational resonance are influenced by zero-quantum line shape parameters including T2 zq and the dispersion in isotropic chemical shift differences. Errors in the estimation of these parameters are often the limiting factor in determining the accuracy of a distance measurement. Here we present a modification of R2, termed “rotational resonance tickling” (R2T), which uses a ramped rf field to induce fast passage through the dipolar resonance condition, thereby greatly reducing the dependence of the spin dynamics on zero-quantum parameters. Extraction of distance information from the resulting exchange curves is approximately a single-parameter fit, with accuracies in model systems that appear to be on the order of ±0.1 Å or better. An additional feature of the technique is that it does not demand the very high-power 1H decoupling fields typically required in other recoupling experiments to limit signal loss during mixing. We demonstrate the technique in a pair of 13C2-labeled model compounds, tyrosine ethyl ester and glycylglycine hydrochloride, with effective internuclear distances (including intermolecular effects) of 5.05 and 4.3 Å, respectively.